The present invention relates to a fuel cell system having (1) an evaporator unit for generating a starting-material gas flow from the preheated fuel/water mixture; (2) a gas generation unit for producing a hydrogen-rich gas containing carbon monoxide from the starting-material gas flow; (3) a gas cleaning unit for selective removal of carbon monoxide from the hydrogen-rich gas; (4) a cooling unit for the gas cleaning unit; and (5) at least one fuel cell comprising an anode chamber through which cleaned, hydrogen-rich gas flows, a cathode chamber through which an oxygen-containing medium flows, and a cooling space through which a coolant flows.
As fuel, conventional fuel cells use hydrogen, which is produced from a mixture of a liquid fuel (for example, methanol), and water in stages connected upstream of the fuel cell. Since the production of the hydrogen usually also entails the formation of carbon monoxide, which is damaging to the fuel cell, a gas cleaning unit is additionally incorporated so that substantially pure hydrogen acts on the anode side of the fuel cell. Further, an evaporator unit is provided for evaporating the fuel and/or water. Finally, to avoid environmental pollution, an exhaust-gas treatment unit is provided, in which all the combustible constituents of the fuel cell exhaust gases are converted as completely as possible.
A fuel cell system of the generic type is known from EP 861 802 A2. In this fuel cell system, all the subsystems, such as a preheater for a fuel/water mixture, an evaporator, a gas generation unit, a gas cleaning unit and exhaust treatment are integrated in a common arrangement of plates. In this case, the gas cleaning unit is arranged adjacent to the preheating unit, for the purpose of dissipating the thermal energy produced during the gas cleaning.
A drawback of this system is the fact that the thermal energy which can be taken up by the fuel/water mixture is not sufficient under all operating conditions to adequately cool the gas cleaning stage, and consequently undesirable temperature peaks still arise in the fuel cell. A further drawback is that the fuel/water mixture is at least partially evaporated as early as at the preheating unit, so that a phase mixture of liquid and gaseous fuel and/or water flows through the preheating unit. However, the heat capacity of the cooling medium is highly dependent on its state, and consequently, under certain circumstances, there is insufficient cooling capacity available in regions with gaseous coolant, and therefore local overheating may occur.
U.S. Pat. No. 5,344,721 discloses a fuel cell system in which fuel and water are supplied separately. In this case, separate preheating units and evaporators are provided for the fuel and the water. Therefore, in this arrangement a large number of components are required, which is unacceptable in particular for mobile applications in view of the space available, costs, and weight.
It is an object of the present invention to provide a fuel cell system which is of compact structure, has improved system efficiency, and ensures a reliable heat balance both for the gas cleaning unit and for the fuel cell.
This object is achieved by a fuel cell system according to the present invention.
The combination of a gas cleaning unit with an associated cooling unit and a heat exchanger connected downstream of the gas cleaning unit for the purpose of preheating the crude fuel/water mixture makes it possible to ensure that, even in the event of load changes, the hydrogen-rich, cleaned gas fed to the anode chamber of the fuel cell is at a temperature which is appropriate for the fuel cell. Unacceptable temperature peaks are therefore avoided. At the same time, this energy which is extracted from the hydrogen-rich, cleaned gas is fed to the crude fuel/water mixture. This reduces the evaporator capacity required and improves the efficiency of the system. At the same time, the load, in particular thermal stresses, on the evaporator unit is reduced by the reduction in temperature gradient. In this way, the service life and the dynamic performance of the evaporator unit can be improved. Compared to systems with separate fuel and water feeds, it is possible to eliminate the need for at least a preheater unit or an evaporator unit.
Making the coolant emerging from the fuel cell act on the heat exchanger has the advantage that the load on the coolant circuit of the fuel cell is relieved, and consequently a second heat exchanger has to dissipate less thermal energy. This heat exchanger can therefore be of smaller design, which is desirable in particular for mobile applications. At the same time, in this arrangement too, the thermal stresses in the evaporator unit are reduced and the efficiency of the system is improved.
Moreover, the coolant circuit of the fuel cell can also be used to cool a further heat exchanger designed as a reformate cooler. In this case, it may be necessary for a further heat exchanger, which is acted on by a cooling medium, also to be provided between the preheating unit and the reformate cooler. With this arrangement too, it is possible to ensure a reliable heat balance both for the gas cleaning unit and for the fuel cell. At the same time, the efficiency of the system is improved by the preheating unit and the load on the evaporator unit is reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.